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Continuous Glucose Monitoring

by Laurie Block, RD, CDE

Blood glucose monitoring has gotten easier, more accurate, and less painful over the years, but it still provides only snapshots of a person’s blood glucose level. Even a person who monitors several times a day really only knows what his blood glucose level is several times a day; the rest of the time, he simply can’t be sure. However, an increasingly popular continuous glucose monitoring device called the CGMS System Gold can help fill in some of those gaps.

The CGMS System Gold is the successor of the CGMS, a Medtronic MiniMed product that was approved for marketing by the Food and Drug Administration (FDA) in 1999. The system measures the glucose level of the interstitial fluid (the fluid between the cells of the body) just under the skin every 10 seconds, then averages the readings over five-minute intervals and stores those averages in memory. (Readings and averages are not displayed in real time.) That adds up to 288 recorded glucose levels during the course of a 24-hour period. Although there may be a slight time lag between changes in blood glucose levels and changes in interstitial glucose levels, the difference is negligible. The CGMS System Gold can be used for up to three days at a time.

The main advantage of a continuous glucose monitoring system such as the CGMS System Gold is its ability to uncover patterns that are missed by conventional blood glucose monitoring. Many people find, for example, that their blood glucose level after meals is much higher than they realized because they were not monitoring when their blood glucose was high. It is also not uncommon for continuous monitoring to reveal previously undetected hypoglycemia during the night—again, a time when most people do not monitor regularly.

Sometimes, the CGMS System Gold is simply worn for three days to see what blood glucose patterns emerge. In many cases, however, a health-care provider will recommend using it to gather information on a specific aspect of diabetes care. For instance, a pump user might use continuous monitoring to assess his basal insulin rates. This is often done by skipping a meal each day to see whether blood glucose levels stay in target range without the influence of food or bolus doses. The CGMS System Gold would give a complete look at what’s happening between the two meals that are eaten. Continuous monitoring can also be used to assess a person’s insulin-to-carbohydrate ratios (the number of grams of carbohydrate one unit of insulin covers at meals). In this case, the person would eat preplanned meals with known amounts of carbohydrate and would inject or infuse preplanned amounts of insulin before meals to see whether the doses of insulin were sufficient to cover the food.

While the CGMS System Gold itself is not approved by the FDA for making insulin dosing decisions, the data it provides can help determine when conventional blood glucose monitoring should detect out-of-range blood glucose levels. Changes in insulin dosing or timing of insulin injections or boluses might then be made on the basis of such monitoring, and the CGMS System Gold could then be used again to evaluate the effects of those changes.

It should be noted that the CGMS System Gold has no direct effect on diabetes control while it is being used; its data can only be viewed and analyzed after the fact. The user must carry out his usual diabetes self-care regimen, including blood glucose monitoring, for blood glucose control while using the system. In fact, to get the most out of continuous monitoring, the user may want to keep more complete records than usual, noting timing and content of meals, timing and doses of medicines or insulin, duration and exertion level of physical activity, and any feelings of stress or unusual occurrences. With detailed records like these, the user can see not just what his glucose level was doing but also what influenced any rises or falls.

Using the system
The CGMS System Gold consists of three components, all of which are extremely compact. They include a small, needlelike sensor that is inserted just beneath the skin, usually on the abdomen; a monitor about the size of a pager that collects and temporarily stores the data from the sensor; and a cable that connects the sensor to the monitor. Insertion of the sensor, which should feel much like the insertion of a needle, is accomplished via a spring-loaded mechanism similar to an insulin pump infusion set inserter. The sensor is held in place by transparent tape.

Each day that it is used, the CGMS System Gold must be calibrated, or compared against a known standard, four times. In this case, the standard is the user’s blood glucose meter, so at least four conventional blood glucose checks, preferably spaced fairly evenly over a 24-hour period, must be done daily and the results entered into the system monitor. (Of course, as many other checks as are necessary to maintain blood glucose control during this time should also be performed.) Blood samples from fingertips must be used for calibration because the device is not approved for calibration with blood samples from alternate sites such as arms or legs. Because blood glucose meter results are being used as the standard, it’s important that they be as accurate as possible. For this reason, it’s a good idea for CGMS System Gold users to review their blood glucose monitoring technique and meter accuracy with a health-care provider before getting started with continuous monitoring.

In addition to entering at least four daily blood glucose monitoring results, the user can (and should) also enter mealtimes, insulin (or other medicine) dose times, and exercise times. These data are stored in the system’s memory and are ultimately integrated into the analysis done at the end of the one- to three-day period. Since the system cannot store more detailed information such as what kinds of foods have been consumed or the type or intensity of exercises undertaken, keeping a daily logbook is recommended.

Because the system is not waterproof, it must be covered with a special shower pack during showers. In addition, insulin should not be administered near the sensor insertion site, and a health-care provider should be alerted if inflammation, infection, or bleeding develops at the site.

After the CGMS System Gold has been worn for the desired period of time, the sensor should be removed, the monitor turned off, and the system returned to the physician’s office. The physician can then upload the data onto a computer via a device known as the Com-Station. Special software designed for use with the CGMS System Gold organizes and presents the information in the form of pie charts and graphs. (Click here for a sample chart.)

The next generation
The CGMS System Gold monitor must be brought to a health-care provider’s office to transfer its data to a computer and analyze it. Another continuous glucose monitoring tool recently released by Medtronic MiniMed, the Guardian Continuous Glucose Monitoring System, allows the user to do that at home. This device consists of a subcutaneous sensor, a transmitter attached to the sensor with a cable, and a monitor that receives glucose readings from the transmitter via radio waves. Like the CGMS System Gold, the Guardian can be worn for up to three days at a time and must be calibrated with periodic conventional blood glucose checks. The Guardian’s monitor can also be set to sound an alarm if glucose levels are rapidly falling or rising out of range. Although glucose levels are not visible on the monitor itself, a docking station and software allow the user to upload and analyze results on a personal computer. Continuous glucose monitoring systems that show real-time glucose levels are in development by several companies.

Filling in the blanks
If you’d like to use the CGMS System Gold to evaluate your diabetes control, speak with your health-care provider. He may have or be willing to obtain a system, or he may be able to refer you to someone familiar with the device. (You can also contact Medtronic MiniMed at [800] MINIMED [646-4633] for assistance with locating a provider in your area.) Additionally, although continuous glucose monitoring with the CGMS System Gold is covered by Medicare and most private insurance companies, it is a good idea to speak to your insurance company ahead of time to find out whether your plan will pay for it.

For more information about the CGMS System Gold or the Guardian Continuous Glucose Monitoring System, call Medtronic MiniMed or visit them online at www.minimed.com.

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